Stirling cycle engines, historically known as "hot air engines", have been developed for cars, buses and industrial applications. Their development, despite their promise of superbly high energy efficiency and low pollutant release, has not met with significant commercial success. This has largely come about because complex pressure seals are needed to contain high pressure gas at high temperature on sliding (reciprocating) shafts. At this time they have found application as engines only in exotic special areas, such as in non-nuclear submarines and in miniature form as cryogenic coolers, eg. for Hubble space telescope electronics. Their theoretic advantage is such that a solution to their heretofore problems would literally change the world energy scenario.
Typically, Stirling engine forms have involved a reciprocating "gas volume displacer" which merely moves a bulk of contained gas back and forth between a hot (externally heated) and a cold (externally cooled) end of an enclosed (usually cylindrical) chamber. This action causes the pressure in the chamber to rise and fall, and the rise and fall of pressure produces power output in an associated power piston. An alternative form of the engine uses multiple, combined power/displacer reciprocating pistons, to effect the volumetric movement of the gas between heated and cooled cylinder heads, and to extract power.
In all apparent known forms of the engine, including multi-cylinder engines with combined power/displacer pistons, crank style mechanisms effect the movement of the displacer and power pistons. These crank or crank-equivalent mechanisms have the property of a near sinusoidal reciprocating motion. This form of motion is utilized advantageously in known forms to cause one element (say the displacer), to have a low or zero motion at one portion of the stroke while the other element (say, the power piston) is moving at high speed. This approximately 90.degree. out-of-phase action is essential to the known principles and forms of the Stirling cycle.
The known principles of the Stirling cycle engine are such that efficiencies and power rise as the contained fluid pressure rises and the temperature between the hot and cold ends rises. Therefore, useful engines favour the use of quite high internal pressures even to 100 atmospheres and hot-end temperatures of 550.degree. C. are routinely used. When such extreme conditions are coupled to the need for non-lubricated reciprocating parts, bearing and seals to contain the power fluid, the need for "cross-heads" or other complex crank mechanisms such as rhombic gear drives to maintain the close tolerance reciprocating seal parts in alignment, then the complexity and the need for sophisticated materials of construction will be readily appreciated. Because the geometry of cylinders tend to minimize external surface (available for heat transfer), compared to the internal working fluid volume, existing known forms of the engine must use complex "nests" of external heater/cooler and heat regenerator tubes, and even then power is usually limited by heat transfer difficulties.
A further practical difficulty has beset the commercialization of the Stirling cycle engine, namely power control. In (say) a gasoline engine, when throttle is adjusted, the change in power output of the engine is almost instantaneous. In a Stirling cycle engine however, adjusting the burner throttle does not instantaneously reduce the temperature of the hot metal mass comprising heat transfer surfaces, so the power change of the engine lags severely. Very complex mechanical schemes have been required in known reciprocating forms of the engine to overcome these effects.
All these complexities have been the barrier to successful commercial application of the known forms of the Stirling cycle engine.